Throughout this application, expanders and compressors will be referred to as “machines”. Seal performance limits are typically categorized by the product of pressure differential across a seal ‘P’ and a seal interface surface velocity ‘V’, known as the ‘PV’ factor for the application. Generally, different seals have differing PV envelopes and limits. These PV envelopes are both seal-type and application specific. One challenge with successful sealing has been operating within the PV limit envelope of the seals, or alternately, finding a seal with a PV limit capacity that is greater than the PV of the particular application. The PV limit is an approximation of the conditions to which a seal will operate, but there may be other factors that influence a seal's capability. Methods for implementing pressure balancing techniques or pressure differential reduction techniques to mechanical sealing arrangements to reduce the PV across seals are also known. For clarity, in this application, pressure balancing or chamber pressure modification will refer to both a reduction in pressure differential across a seal as well as to equalizing pressure across a seal. The below will look to describe a unique combination of pressure balancing seals and pressure cascading approaches to these principles. Use of lip seals, labyrinth seals, mechanical face seals or dry gas seals within expanders and compressors are necessary in “dry” expanders and compressors to keep lubricant separate from process, and are common in rotating equipment.
In addition to PV limits, heat removal from the seal is a critical component to a sealing solution. Often, lubricant that is used to lubricate bearings, gears and other rotating equipment is also circulated onto the seal to assist with lubrication and to remove heat from the seal. That lubricant is then typically externally cooled in a heat exchanger and recirculated back into the rotating equipment. Typically, lubricant circulation systems are vented to atmosphere and do not operate as a pressurized system, under a pressure blanket as will be described below.
In many instances, different types of mechanical shaft seals can accomplish the same PV objective, but may provide other different advantages. Throughout this application, mechanical seals, including but not limited to lip seals, mechanical face seals, dry gas seals, and labyrinth seals, will be referred to as “seals”. Generally, lip seals have a lower PV than mechanical seals and are typically better suited for higher speeds (higher V) and lower pressure differentials (lower P) whereas mechanical face seals are typically better suited for lower speeds (lower V) and higher pressure differentials (higher P), although each seal type has its limits whether that be with pressure differential or seal interface speeds. Dry gas seals, also known as non-contacting or dry-running seals, are best suited for sealing gases at high speeds with high pressure differentials where sealing of a vapor is required. Dry gas seals are non-contacting, dry-running mechanical face seals. Labyrinth seals are another (non-contacting) mechanical seal that utilize the principle of providing a difficult and obstructive path for the gas or fluid being sealed. Each step in a labyrinth seal creates eddies which then retard the flow from the pressure differential across the seal. These seals are particularly useful in rotating equipment having high rotational speeds, because they are non-contacting and therefore do not have much, if any, friction.
Methods for implementing magnetic couplings are common in pumps and have been described in scroll compressor applications, typically used in refrigeration systems. Although magnetic coupling use with pumps were initially developed to hermetically seal the pump and the contents of the pump from the drive mechanism, often not to contaminate the product in the pump, the basic principles of isolating the drive from the pump can also be utilized with an expander or compressor to contain vapor from escaping. An example of a magnetic coupling used on an expander would be in a pressure let down expander or from an expander in use with an organic Rankine cycle (ORC) system. In an ORC system, a dry expander would require a lubricating pump to circulate lubricant to the expanders bearings, gears and seals. In the system described below, a process fluid pressure blanket is applied to the lubricant circulation system, and the use of magnetic couplings on the lubricant circulation system pump(s) provides an advantage of not developing leaks to the process through the lubricant circulation system, specifically through the seals on the lubricant pumps.
Shaft seals are typically not required on oil-injected (aka oil-flooded) screw expanders or screw compressors, since the bearings and rotors are lubricated by either delivering lubricant to the bearings, from where it flows to the rotors and then to the exit from the machine, or by mixing the lubricant directly into the process fluid, which then circulates lubricant with the process fluid and lubricates the bearings and rotors as it passes through the machine. The lubricant is then either circulated throughout the system continuously (such as in refrigeration systems), or the lubricant is separated from the process fluid on exit from the rotating equipment machine in a coalescor/separator/mist eliminator (as is done in some natural gas compression applications). Once separated, the lubricant is then re-injected into the bearings and rotors to flow through the expander or compressor. In situations where the process fluid is to remain clean of lubricant contamination, a dry screw machine is a good option because the lubricant is kept separate from the process fluid by mechanical seals. Although a coalescor on an oil-flooded machine may be able to remove almost all of the lubricant, trace amount will remain in the process stream and therefore can contaminate or build up over time in a closed loop process. Contamination is undesirable in processes such as compressing breathing air or in gas separation processes that require a purity to the product being compressed. In pressure let down applications, the pipeline company would find it undesirable to end up with lubricant in their pipeline system and in ORC systems, a build-up can occur and a film of lubricant may develop on the heat exchangers. This lubricant would then reduce the thermal conductivity of the heat exchangers and reduce the overall efficiency of the ORC system. Although refrigeration systems operate almost exclusively with oil-flooded machines, an opportunity may exist to use a dry compressor in refrigeration application. Therefore, an option for a dry expander and compressor has its applications.
Should the machine require a gear box, such that the final drive shaft has a different speed from the machine's rotor shaft, then an integral gear box can demonstrate advantages to an external gear box by applying pressure balancing that will be described below. With an integral gear box, the sealing of the final drive shaft could be sealed with one or more seals in a dynamic sealing arrangement that can wear over time and thereby allow ambient air to make its way into the machine housing or allow working vapor out of the machine to atmosphere. A double mechanical seal is extremely well suited for sealing a final drive shaft. The double mechanical seal can be arranged face-to-face or back-to-back (it is also known as a dual pressurized and dual unpressurized mechanical seal, depending on whether the space in between the two seals is pressurized or not). Throughout this application, a double mechanical seal, whether pressurized or not will be referred to as a “double seal”. The advantages of a double seal is that it provides two layers of protection from leaking the system charge to atmosphere as well as provide a visual indication to an operator when the outer seal has started leaking. When the inner seals on a pressurized double seal that is using the systems lubricant to pressurize the double seals cavity leaks, fluid level in the chamber will rise above its normal level thereby indicating the seal is leaking. Should the outer seal leak, it will become visible to an operator and the seal will garner the required attention. Should the machine require an integral gear box, such that the final drive shaft has a different speed from the machines rotor shaft, then a combination of seals, double seals, or a magnetic coupling can be affixed to either the rotor shaft of the machine or the final drive shaft of the integral gear box appended to the expanders/compressors shafts. Similarly, should an integral gear box not be required, the seals, double seals or magnetic coupling can be affixed to the rotor shaft or the machine.
Another solution to prevent loss of process fluid could be the integration of a magnetic coupling to the output shaft. This arrangement is desirable as it: 1) preserves the process fluid thereby saving money, 2) prevents air from entering the machine housing and thereby preventing contamination of the process fluid, and 3) makes the system inherently safe by preventing leaks of potentially hazardous, flammable or explosive process fluids. Magnetic coupling technology has no theoretical size limit but there is a size limit to commercially available magnetic couplings, and therefore the size of machine that becomes coupled with a magnetic coupling is limited in size. Further, magnetic couplings for a fixed physical size have a torque limit before they start slipping and therefore, higher speed with lower torque output shafts can extend the range of a particular size of magnetic coupling, rather than lower speed, higher torque shafts.
A magnetic coupling could also be utilized on the lubricant pump(s) so that the pump(s) can operate at pressures higher than the balancing process fluid vapor. The pump(s) will require the capability of operating with a positive head pressure from the balancing process fluid vapor, in addition to adding a pressure differential to the lubricant so that it will flow to lubricate the various components in the system. To overcome this challenge, replacing pump seals with a magnetic coupling allows the machines lubricant circulation system to operate at higher than normal pressures while at the same time eliminating the risk of a leak point.
Ambient air can make its way into the expander when the pressure on the machine side of a seal is less than atmospheric pressure. This condition can materialize when: 1) the process fluid cools to create a negative pressure and therefore placing the machine chamber in vacuum, drawing ambient air into the equipment, or 2) when the machine undergoes a rapid slow down due to a change in high pressure throttle valve position, negative pressure can be effected on the shafts seals thereby allowing ambient air to make its way past the seals into the process fluid. In closed loop systems such as refrigeration systems and organic Rankine cycle systems, entry of ambient air into the system pollutes the process fluid and reduces the systems efficiency due to non-condensables entering the sealed system. For any type of seal to be effective, it has to leak a nominal amount. These nominal amounts are designed to be minimal.
Another suitable approach to sealing lubricant from the process, and sealing the process from atmosphere, is to design the seals to leak. The direction of leaking can be designed into the system such that the process leaks from the process to the direction of atmosphere. The process fluid that does get past the seal and into the lubricant circulation system can then be separated from the lubricant and circulated back into the process at a lower pressure point in the system, and the lubricant drained back into the lubricant circulation system. Although some efficiency is lost by allowing the process fluid to bleed through the seals and around the machines internal components, it is an effective approach.
U.S. Pat. No. 4,328,684 describes a method for using a magnetic coupling between a twin screw compressor and a twin screw expander for use in a refrigeration system. A wall between the compressor housing and the expander housing contains a magnetic drive that is connected to rotate with the output shaft of the compressor and input shaft of one of the expander rotors so that power can be transferred between the two. US Publication number: US 2011/0176948 describes a scroll expander coupled with a magnetic coupling. However, the configurations described therein have numerous drawbacks, and a need therefore exists in the art to find expander sealing solutions that can handle high operational PV ratios and provide inexpensive sealing arrangements in a number of different applications.
The techniques described herein may apply to any number of processes incorporating expanders, including but not limited to Organic Rankine Cycle (ORC) processes, and gas pipeline or steam pressure letdown applications. In the case of compressors, including but not limited to refrigeration, natural gas compression, and air compressor applications. Advantageously, the present techniques may be applied to ORC processes and pressure let down expander applications. More advantageously, the disclosed techniques may be applied to ORC processes. Even more advantageously, the disclosed techniques may be applied to ORC recovering waste heat and/or ORC systems utilizing one or more twin screw expanders. It is also possible, as in the case of a condensing expander, that the processes fluid leaving the expander or the last of a series of expanders may be either a semi-saturated vapor or a liquid.